[MCLENNAN] EARTH'S PENETRATING RADIATION 9 
The readings given in Table IL it will be seen with the exception 
of that obtained on August 18th, vary by not more than 3 per cent 
from their mean value, while those given in Table III were the same, 
namely, 8-32 ions per ce. per sec. for measurements made between 
10 o’clock in the morning and half past two in the afternoon, the rate 
of movement of the gold leaf in all five of these observations being 
constant. In so far as these observations go, therefore, it seems certain 
that variations in the intensity of the penetrating radiation at the 
observation stations selected at Cambridge and in Scotland, do not 
exist in amount greater than 2 or at most 3 per cent. 
This result is in practical agreement with what had previously been 
found to characterise the radiation at Toronto, and this characteristic, 
taken in conjunction with the low value found for “q” over the waters 
of Lake Ontario is what led the writer to conclude that the soil of the 
earth and not its atmosphere nor any of the celestial bodies was the 
chief source of the penetrating radiation. 
In conclusion I wish to acknowledge my indebtedness to my assist- 
ants, Mr. P. Blackman and Miss A. T. Reed, for making some of the 
reductions and for taking a number of the readings at Toronto. 
Physical Laboratory, 
University of Toronto. 
May Ist, 1912. 
Sec. ITI., 1912. 2 
